[ARC5] S+N/N ratio results.
Bob Macklin
macklinbob at gmail.com
Fri Jun 14 18:05:24 EDT 2013
The only thing wrong on using the audio VTVM on other than a 600ohm line is
the measurement of power.
Otherwise they are just a high Z AC VTVM.
Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
----- Original Message -----
From: "Geoff" <geoffrey at jeremy.mv.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
Cc: "ARC5" <arc5 at mailman.qth.net>
Sent: Friday, June 14, 2013 2:22 PM
Subject: Re: [ARC5] S+N/N ratio results.
>
>>>I think these radios do just fine with weak signals on 40M
>>>or lower where
>>> the atmospheric noise is higher. I put a passive DBM in front of a
>>> BC-453B
>>> and it will capture whatever is above band noise. Stability is good too.
>>> The issue is CW selectivity. You get used to what we did as novices,
>>> use
>>> the analog signal processor between our ears.
>>>
>>> Dennis AE6C
>>>
>>
>> I think it depends on the particular receiver. There are other
>> characteristics beside noise that affect the performance at higher
>> frequencies. One is the losses in the coils and condensers and other
>> insulators. The material of the tube sockets and bases can contribute.
>> Conventional signal to noise is measured as follows:
>> Instruments needed; A signal generator with a calibrated attenuator
>> capable of putting out very low levels down to a fraction of a
>> micro-volt, and with very low leakage, adequately shielded cable to
>> attach the generator to the receiver. Some means of verifying the voltage
>> level at the receiver; an audio frequency volt meter. This will be
>> connected at an audio output so does not need high sensitivity.
>
>
> ** Since most audio meters are calibrated for 500-600 Ohms there is a
> mismatch to many radios.
> I use an overhauled GR 583A to match most anything and eliminate those
> errors.
>
>
>> Most receivers that have a specification for signal to noise also
>> specify a dummy antenna or a source impedance for the measurement. The
>> voltage at the generator terminal needs to be known if the measurement is
>> to be meaningful.
>
>
> ** Receivers with screw terminal or single wire antenna inputs will not
> give accurate results, especially on higher frequencies. The best you can
> do there is use a shielded balun with as close to zero length output leads
> as possible. Even then there is some leakage/stray coupling. I also wrap
> the input coax many times thru a 2.4" 43 Mix toroid to minimize common
> mode leakage. See below for impedance concerns.
>
>
> Some
>> receivers have fairly constant input impedance, some do not but specify a
>> dummy antenna to allow for reproduceable measurements.
>
>
> ** Even those with coax input connectors can have a 2 to 4:1 mismatch if
> the receiver is general coverage. The various dummy antennas do a decent
> job for a simple military type PM and alignment since the manual is
> matched to the required test equipment and the "close enough for
> government work" rule takes precedence.
> Getting a perfect SNR reading would require a using a bridge, VNA, MFJ
> 259B or similar, to locate that almost perfect 50 Ohm input impedance spot
> frequency (with or without a balum, dummy antenna or whatever) at which to
> take the measurement. This is a waste of time to most where an
> "acceptable" average is sufficient.
>
> ** An alternative, and good idea in all measurements, is to force a 50 Ohm
> input and output with a 6dB minimum loss pad made from carbon film
> resistors if you dont have a commercial version. Any value between 6-10dB
> is fine and you can use an online calculator to determine attenuation
> using standard resistor values.
>
>
>> The generator is modulated 30% at (usually) 400 hz. Preferably the
>> generator should be capable of low distortion modulation with low
>> incidental FM so that the sidebands are symmetrical.
>
>
> ** This eliminates the usual "service grade" kits and other low end test
> gear many use and who then take results as gospel and not to be disputed.
> They are also huge harmonic generators to further result in poor accuracy.
>
>
>> We will assume the measurement is for a 10 db signal to noise ratio.
>> If for some other value set the levels below accodingly.
>> The audio meter is attached to the receiver. AVC is turned off and RF
>> gain is set at maximum. This is not changed.
>
> ** OK
>
> A reference level is chosen for the noise,
>> preferably a level where there is no hum or other audio noise but enough
>> audio dynamic range to allow at least a 10 db increase.
>
>
> ** Since the meter calibration is likely non linear pick a pair of 10dB
> mid range points that is easy to interpolate small changes.
>
>
>
> Set the generator for a low value of output
>> with no modulation, say one microvolt. Tune and peak the receive
>> carefully and set the audio gain for the reference you have chosen to be
>> zero level. turn on the modulation. The output will go up. Adjust the
>> receiver audio gain for an output exactly 10 db above the noise reference
>> (for a 10 db ratio). Now, turn the modulation off again and read the
>> noise. If not still at zero (probably will not be) adjust the _generator_
>> output level to bring the noise back to the reference. Now start the
>> modulator again and adjust the receiver AF gain for ten db above the
>> noise again. You may have to juggle these two several times to get it to
>> where the level with modulation is exactly 10 db higher than the noise.
>> When you acheive this read the level of the RF output from the attenuator
>> or meter on the generator. This is the RF level for 10 db Snr.
>
> ** I find it far easier to leave the modulation on and tune the receiver
> on and off the signal since the receiver is likely still drifting on the
> higher frequencies. I first set the noise only reference on the meter.
> The above procedure is fine for stable receivers but still requires
> considerable arm movement.
>
>
> This level is likely to change with
>> frequency depending on receiver design. It will also change with IF
>> bandwidth and is meaningless without a statement of the bandwidth.
>
>
> ** As long as one accurate test is made the data for other bandwidths can
> be calculated.
>
>
> It will also change somewhat with the shape
>> of the IF filter being less for a given bandwidth if the skirt
>> selectivity is greater.
>> Many receivers give specs for a 10db Snr but not all. For instance,
>> the RCA AR-88 gives one for 6db and one for 20 db.
>> Also note that many receivers give only a minimum RF level for a
>> minimum audio output level. This is meaningless other than being a
>> simple test of of the receiver's condition.
>
> ** Typical marketing BS
>
>> Typical 10db signal to noise ratios with typical IF bandwidths will
>> vary from around 5uv to less than 1 uV partly due to the variation in
>> bandwidth. Older receivers with fixed bandwidths will vary from around 6
>> khz down to around 3 khz.
>
>
> ** More like 10-15 kc at the 6dB points with the typical 2 IF stage, 4
> tuned 455kc circuits receiver. AKA broad as a barn door.
>
>
> If the SNR is measured at seveeral
>> frequencies over the range of the receiver it will give some idea of the
>> consistency of performance. Very good receivers will not vary much.
>
>
> ** The usual 4-5 band .5 to 30mc receiver can have a fairly wide variation
> between band edges but since they overlap, one or the other will be
> better. Ham band only have tighter tolerance wheras some of those 30 band
> sets can have quite a bit of variation...think Collins.
>
>
>
>> Single conversion receivers may show higher noise at high frequencies
>> because one is seeing the image noise as well as the noise at the main
>> signal tuning. This is probably significant only for single RF receivers
>> with low IF frequencies.
>
>
> ** Single IF's of later years were often at 5 to 9 mc and images werent an
> issue at HF. A 455 kc IF and a 1 to 5 uV input should not have any
> measurable image noise if it has at least one RF stage. Even those popular
> no RF stage entry level sets likely have a 8-10dB image rejection at 30
> mc.
>
>
>> My AR-88, which is a remarkable receiver for its time, requires less
>> than 1 uV for 10db SNR right up to 30 Mhz using the widest crystal filter
>> setting, about 3 khz.
>> I've measured a number of other receivers this way but have misplaced
>> my notes. By memory the SP-600 did about 2 uV for the 3 khz bandwidth.
>> It should have done as well as the AR-88 but its possible the bandwidth
>> of the AR-88 is narrower. However, the AR-88 gave 1.8 uV at the 8 khz
>> bandwidth setting so I think perhaps its actually pretty quiet.
>
>
> ** For decent AM all filters should be OFF even tho the numbers often are
> not very complimentary.
>
>
>> I stress again that this method does not produce a true noise
>> measurement. Its mostly useful for trouble shooting. A true measurement
>> would measure _Noise Figure_ which is a meaasure of the noise from a
>> receiver or amplifier in comparison to the theoretical minimum noise.
>> That is, the ratio in db of the noise output of a practical circuit to
>> the noise from an ideal noisless circuit. Noise figure is measured using
>> a noise generator. Noise generators can be made so that the noise output
>> can be known very accurately. The measurement is simply a reading of the
>> output on the generator to the output with a proper termination on the
>> amplifier or receiver. The ratio in db is the N.F.
>> Hewlett-Packard published an application note, actually a book on N.F.
>> measurement which is available on line from:
>> http://www.hparchive.com/appnotes.htm
>> look for AN-57-1 and AN-57-2 while these are oriented toward microwave
>> applications the theory holds for lower frequencies as well.
>
>> Richard Knoppow
>> Los Angeles
>> WB6KBL
>> dickburk at ix.netcom.com
>
> ** Ive been using a NF meter ever since I built a Sylvania 5722 vacuum
> tube diode version in the 60's. These day I use a HP 8970A with noise
> head that I bought at a belly up company auction in 2002. I used them for
> so many years at various jobs I could likely program it in my sleep (-;
> Carl
> KM1H
>
>
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